The invention relates to a method of regenerating spent hydrocarbon conversion catalyst by the combustion of coke on the catalyst in a fluidized combustion zone. This invention specifically relates to a process for the conversion of heavy hydrocarbons into lighter hydrocarbons with a fluidized stream of catalyst particles and regeneration of the catalyst particles to remove coke that acts to deactivate the catalyst.
Fluidized catalytic cracking (FCC) is a hydrocarbon conversion process accomplished by contacting hydrocarbons in a fluidized reaction zone with a catalyst composed of finely divided particulate material. The reaction in catalytic cracking, as opposed to hydrocracking, is carried out in the absence of added hydrogen or the consumption of hydrogen. As the cracking reaction proceeds substantial amounts of highly carbonaceous material referred to as coke is deposited on the catalyst. A high temperature regeneration within a regeneration zone operation burns coke from the catalyst. Coke-containing catalyst, referred to herein as spent catalyst, is continually removed from the reaction zone and replaced by essentially coke-free catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams allows the transport of catalyst between the reaction zone and regeneration zone. Methods for cracking hydrocarbons in a fluidized stream of catalyst, transporting catalyst between reaction and regeneration zones, and combusting coke in the regenerator are well known by those skilled in the art of FCC processes. To this end the art is replete with vessel configurations for contacting catalyst particles with feed and regeneration gas respectively.
A common objective of these configurations is maximizing product yield from the reactor while minimizing operating and equipment costs. Optimization of feedstock conversion ordinarily requires essentially complete removal of coke from the catalyst. This essentially complete removal of coke from catalyst is often referred to as complete regeneration. Complete regeneration produces a catalyst having less than 0.1 and preferably less than 0.05 wt-% coke. In order to obtain complete regeneration, the catalyst has to be in contact with oxygen for sufficient residence time to permit thorough combustion.
Conventional regenerators typically include a vessel including a spent catalyst inlet, a regenerated catalyst outlet and a distributor for supplying air to the dense bed of catalyst that resides in the vessel. Cyclone separators remove catalyst entrained in the spent combustion gas before the gas exits the regenerator vessel. U.S. Pat. No. 4,610,851 discloses a regenerator vessel with two air distributors at different levels to assure adequate distribution of combustion gas throughout the vessel. U.S. Pat. No. 5,827,793 teaches at least two air distributors at different levels in the lower half of the dense bed of catalyst to promote a reducing environment in dense bed. U.S. Pat. No. 4,843,051 show two air distributors grids at different levels in a regeneration vessel to assure adequate combustion. U.S. Pat. No. 5,773,378 teaches a regenerator vessel with a lower air distributor and air enters above the lower air distributor with spent catalyst.
In a dense catalyst bed, also known as a bubbling bed, combustion gas forms bubbles that ascend through a discernible top surface of a dense catalyst bed. Relatively little catalyst is entrained in the combustion gas exiting the dense bed. The superficial velocity of the combustion gas is typically less than 0.3 m/s (1.0 ft/s) and the density of the dense bed is typically greater than 640 kg/m3 (40 lb/ft3) depending on the characteristics of the catalyst. The mixture of catalyst and combustion gas is heterogeneous with pervasive gas bypassing of catalyst.
One way to obtain fully regenerated catalyst is by performing the regeneration in stages. U.S. Pat. No. 3,958,953 describes a staged flow system having concentric catalyst beds separated by baffles which open into a common space for collecting spent regeneration gas and separating catalyst particles. U.S. Pat. No. 4,299,687 teaches the use of a staged regenerator system having superimposed catalyst beds wherein spent catalyst particles first enter an upper dense fluidized bed of catalyst and are contacted with regeneration gas from the lower catalyst bed and fresh regeneration gas. After partial regeneration in the first regeneration zone, catalyst particles are transferred by gravity flow into a lower catalyst bed to which is charged a stream of fresh regeneration gas. U.S. Pat. No. 4,695,370 and U.S. Pat. No. 4,664,778 disclose two stage regenerators in which each stage is performed in a separate vessel.
The use of relatively dilute phase regeneration zones to effect complete catalyst regeneration is shown in U.S. Pat. No. 4,430,201, U.S. Pat. No. 3,844,973 and U.S. Pat. No. 3,923,686. These patents teach a lower dense bed to which combustion gas is distributed and an upper transport zone. Additional air is distributed in a riser providing the transport zone. A two-stage system that combines a relatively dilute phase transport zone without a lower dense bed zone for regenerating catalyst is shown in U.S. Pat. No. 5,158,919 and U.S. Pat. No. 4,272,402. These patents all teach an upper dense bed into which the at least partially regenerated catalyst exiting from the transport zone collects.
Dilute or transport flow regimes are typically used in FCC riser reactors. In transport flow, the difference in the velocity of the gas and the catalyst is relatively low with little catalyst back mixing or hold up. The catalyst in the reaction zone maintains flow at a low density and very dilute phase conditions. The superficial gas velocity in transport flow is typically greater than 2.1 m/s (7.0 ft/s), and the density of the catalyst is typically no more than 48 kg/m3 (3 lb/ft3). The density in a transport zone in a regenerator may approach 80 kg/m3 (5 lb/ft3). In transport mode, the catalyst-combustion gas mixture is homogeneous without gas voids or bubbles forming in the catalyst phase.
Intermediate of dense, bubbling beds and dilute, transport flow regimes are turbulent beds and fast fluidized regimes. In a turbulent bed, the mixture of catalyst and combustion gas is not homogeneous. The turbulent bed is a dense catalyst bed with elongated voids of combustion gas forming within the catalyst phase and a less discernible surface. Entrained catalyst leaves the bed with the combustion gas, and the catalyst density is not quite proportional to its elevation within the reactor. The superficial combustion gas velocity is between about 0.3 and about 1.1 m/s (1.0 and 3.5 ft/s), and the density is typically between about 320 and about 640 kg/m3 (20 and 40 lb/ft3) in a turbulent bed.
Fast fluidization defines a condition of fluidized solid particles lying between the turbulent bed of particles and complete particle transport mode. A fast fluidized condition is characterized by a fluidizing gas velocity higher than that of a dense phase turbulent bed, resulting in a lower catalyst density and vigorous solid/gas contacting. In a fast fluidized zone, there is a net transport of catalyst caused by the upward flow of fluidizing gas. The catalyst density in the fast fluidized condition is much more sensitive to particle loading than in the complete particle transport mode. Therefore, it is possible to adjust catalyst residence time to achieve the desired combustion at the highly effective gas-solid, mixing conditions. From the fast fluidized mode, further increases in fluidized gas velocity will raise the rate of upward particle transport, and will sharply reduce the average catalyst density until, at sufficient gas velocity, the particles are moving principally in the complete catalyst transport mode. Thus, there is a continuum in the progression from a fluidized particle bed through fast fluidization and to the pure transport mode. The superficial combustion gas velocity for a fast fluidized flow regime is typically between about 1.1 and about 2.1 m/s (3.5 and 7 ft/s) and the density is typically between about 48 and about 320 kg/m3 (3 and 20 lb/ft3).
U.S. Pat. No. 4,849,091, U.S. Pat. No. 4,197,189 and U.S. Pat. No. 4,336,160 teach a riser combustion zone in which fast fluidized flow conditions are maintained. The latter of these patents teaches a combustor regenerator in which complete combustion occurs in a fast fluidized riser zone without the need for the addition of combustion gas to the bed collected from the top of the riser.
A combustor is a type of regenerator that completely regenerates catalyst in a lower combustion chamber under fast fluidized flow conditions with a relatively small amount of excess oxygen. A riser carries regenerated catalyst and spent combustion gas to a separation chamber wherein significant combustion occurs. Regenerated catalyst in the separation chamber is recycled to the lower combustion phase to heat the spent catalyst about to undergo combustion. The regenerated catalyst recycling provides heat to accelerate the combustion of the lower phase of catalyst. Combustors are advantageous because of their efficient oxygen requirements.
As greater demands are placed on FCC units, combustor vessels are being required to handle greater catalyst throughput. Greater quantities of combustion gas are added to the combustor vessels to combust greater quantities of catalyst. As combustion gas flow rates are increased, so does the flow rate of catalyst between the combustion and separation chamber increase. Hence, unless the combustion chamber of a combustor vessel is enlarged, the residence time of catalyst in the lower zone will diminish, thereby decreasing the thoroughness of the combustion that must be achieved before the catalyst enters the separation chamber.